Cardiovascular diseases constitute the leading cause of death in the United Sates regardless of gender or ethnicity. Of these diseases, congestive heart failure (CHF) is the only one that is increasing in prevalence (Massie and Shah 1997; Packer and Cohn 1999). According to the American Heart Association, the number of hospital discharges and the number of deaths due to CHF both rose roughly 2.5-fold from 1979 to 1999. Currently, about 5 million Americans have been diagnosed with CHF, and about 550,000 new cases occur annually (American Heart Association 2001). This life-threatening condition is accompanied by great financial impact. In fact, it is the single largest Medicare expense (Kayser 2002). Direct and indirect costs for treating CHF have been estimated as high as $56 billion (Hussar 2002). Hospital expenses for the treatment of HF are more than double those for all forms of cancer combined (O'Connell and Bristow 1994).
CHF is a common cause of death, is accompanied by high indirect costs for treatment, and has a high mortality rate. Once a patient has been diagnosed with CHF, the one-year mortality rate is about 20% (American Heart Association 2001). The probability for readmission for the same condition is very high, and several studies of readmission have recently been performed (Chin and Goldman 1997; Krumholz, Parent et al. 1997; Krumholz, Chen et al. 2000). Readmission rates in excess of 35% within one year of diagnosis are typical (Tsuchihashi, Tsutsui et al. 2001). Such frequent recurrence results in multiple emergency care visits and inpatient hospitalizations (Krumholz, Parent et al. 1997). Multiple hospitalizations and inadequate therapeutics define the current situation faced by those who suffer from CHF.
A recent randomized study indicated that home-based intervention can potentially decrease the rate of readmission, prolong survival, and improve the quality of life for patients with CHF (Stewart, Marley et al. 1999). In an independent study that looked at socioeconomic factors, Tsuchihashi, et. al. concluded that both outpatient and home-based care are needed in order to reduce the mortality rate and lower the overall costs associated with CHF (Tsuchihashi, Tsutsui et al. 2001). Clearly, new therapies with broad application that can be used on an outpatient basis are desperately needed in this growing market.
Brain type natriuretic peptide (BNP) is one of a family of peptides that are involved in cardiovascular, renal, and endocrine homeostasis. It was discovered in 1988 (Sudoh, Kangawa et al. 1988), almost a decade after the discovery of atrial natriuretic peptide (ANP). Although it was first isolated from porcine brain, it is known for its activity at receptors in vascular smooth muscle and endothelial cells. BNP is an endogenous peptide produced by the heart. It is first produced as prepro-BNP and is subsequently shortened twice to the active form, a 32-amino acid peptide with one disulfide bond.
As illustrated in FIG. 1, BNP binds to the natriuretic peptide receptor A (NPR-A), a membrane bound protein on the cell surface. The binding event triggers the synthesis of cGMP in the cytosol by guanylate cyclase. It is through this secondary messenger that BNP accomplishes the cardio-vascular, renal, and endocrine effects with which it is associated. Regulation of BNP is accomplished by several different means. BNP molecules that bind to NPR-A and stimulate cGMP production are removed from circulation, but there are other means by which BNP is eliminated without invoking a response. The most common means of removal is through binding to the clearance receptor, natriuretic peptide receptor C (NPR-C). Upon binding to NPR-C, the peptide is taken into the cell and cleaved enzymatically. The next major means of clearance is degradation by neutral endopeptidase (NEP), which is a membrane-bound enzyme on the cell surface. Finally, BNP is removed to a small extent by renal filtration.
Under normal conditions, BNP is produced in low amounts in the atria and ventricles. However, when the ventricles are stretched during cardiac decompensation, the amount of BNP that is produced increases greatly. Although the atria are still involved, the ventricles become the main site of production. The heart produces BNP in response to a stretching of the ventricles that occurs during decompensation at the outset of CHF. The effects of BNP include natriuresis, diuresis, vasodilation, and a lowering of diastolic blood pressure. These effects are brought about through the actions of a secondary messenger, cyclic guanosine monophosphate (cGMP). Production of cGMP is triggered when BNP interacts with the natriuretic peptide receptor A (NPR-A) which is a membrane-bound receptor located on the surface of endothelial cells in blood vessels, kidneys, and lungs. Plasma concentration of BNP incrementally increases with increased severity of CHF. Despite this increase, the beneficial effects of BNP are blunted in severe CHF, raising the possibility of a relative deficiency state in overt CHF. Alternatively, as the assays currently employed to measure plasma concentration of BNP do not specifically differentiate between pre-pro BNP and the mature form, this pro-hormone may not be adequately processed to its mature form in overt CHF. Therefore, either the amount of BNP that the heart can produce is overcome or prepro-BNP is not adequately converted into its active form, thus reducing its beneficial actions. Because of its early production at the onset of heart disease, BNP has become important as a diagnostic marker to detect patients who are at high risk of developing CHF (Yamamoto, Burnett et al. 1996; McDonagh, Robb et al. 1998; Richards, Nicholls et al. 1998; Nagaya, Nishikimi et al. 2000; Kawai, Hata et al. 2001; Maisel, Krishnaswamy et al. 2002; McNairy, Gardetto et al. 2002).
BNP functions to relieve cardiac decompensation in several ways. As the name implies, BNP leads to the excretion of sodium and an increase in urine output, which lessen congestion. It also functions as a vasodilator, the effects of which are enhanced by several other actions. Most notable of these functions are the roles BNP plays in the interference of the renin-angiotensin-aldosterone system (RAAS). It leads to inhibition of renin, which is a key enzyme in the generation of the vasoconstrictive peptide angiotensin. It inhibits the overgrowth of epithelial cells lining vascular tissue, which left unchecked, can greatly reduce blood flow. A final way that BNP functions to relieve cardiac decompensation is its lusitropic effects. It improves myocardial relaxation of the ventricles, resulting in lower diastolic blood pressure.
Practical limitations exist in using peptides as drugs. Proteolysis, both in the gut and in the bloodstream, is a major barrier to using peptides as therapeutics. Another difficulty encountered with non-endogenous peptides is immunogenicity. As a result of these problems, the approach of the pharmaceutical industry has been to create small, non-peptide molecules using medicinal chemistry. While this approach has met with success, it is costly, time consuming, and fraught with uncertainty in terms of pharmacokinetics and toxicity. Furthermore, identification of small organic molecules with agonist activity at peptide receptors has proved exceptionally challenging.
While the use of “PEGylated” proteins is well established to date, they have been confined to injectable use. The present invention provides orally available conjugates of polypeptides, such as human brain-type natriuretic peptide (hBNP). Specifically, the present invention provides conjugates comprising PAG linked to therapeutic peptides and proteins in a formulation in the treatment of congestive heart failure. These preparations then function to protect the hBNP against proteolytic enzymes, and thereby permit the effective use of this agent as an agonist of human natriuretic peptide receptor A. As a result of this agonistic activity, there is enhanced production of cGMP.
In August 2001, hBNP (native peptide) was approved by the FDA under the trade name Natrecor® (Nesiritide) for the treatment of acute congestive heart failure. Natrecor® was the first drug approved for the treatment of CHF in over twelve years. It is administered by intravenous continuous infusion over a period of 48 hours. As the drug is expensive and requires hospitalization, Natrecor® is only used for the most acute cases. Despite this expense and inconvenience, Natrecor® may be considered less expensive than some other therapies by reducing the amount of time patients spend in intensive care units.
Currently, almost 5 million Americans have CHF and over 550,000 new cases are reported each year (American Heart Association 2001). Currently, direct costs for the treatment of CHF are well over $20 billion (American Heart Association 2001). With diagnostic procedures now available to detect the onset of heart failure before cardiac damage occurs, there is great need for a drug with expanded utility that can be used in an outpatient or home-based setting.